Exchange mailbox quota management

Managing mailbox quotas in Exchange isn’t glamorous, but it’s one of those things that quietly keeps your environment sane. I’ve had my fair share of “mailbox full” complaints over the years—usually right before a quarterly report or some critical client email. So yeah, I’ve learned to stay ahead of it.

Why I Started Paying Attention to Quotas

Back when I was running Exchange 2013 on a couple of aging Dell R720s, I didn’t bother much with quotas. We had generous storage and a small team. But once we scaled and moved to Exchange 2016 (and later hybrid with Exchange Online), mailbox bloat became a real issue. One user had a 48GB mailbox—no archiving, no cleanup, just years of attachments and newsletters. That’s when I started enforcing quotas seriously.

How I Set Quotas (and What I Actually Use)

I usually start with the Exchange Admin Center (EAC) for quick changes, especially when I’m already in the UI doing other mailbox tweaks. But for bulk updates or scripting, PowerShell is my go-to.

EAC Method (Good for One-Offs)

• Head to Recipients > Mailboxes
• Pick the user
• Hit Edit, then go to Mailbox Usage
• Click More Options and set your thresholds

I typically set:

• IssueWarningQuota at 24GB
• ProhibitSendQuota at 25GB
• ProhibitSendReceiveQuota at 25GB

PowerShell Method (Faster for Bulk)

Set-Mailbox user@domain.com -IssueWarningQuota 24GB -ProhibitSendQuota 25GB -ProhibitSendReceiveQuota 25GB

To double-check:

Get-Mailbox user@domain.com | Select *quota*

Not gonna lie, I’ve mistyped “ProhibitSendReceiveQuota” more times than I’d like to admit. Ever spent an hour debugging a typo? Welcome to my world.

Gotchas and Weird Behavior

One thing I learned the hard way: quotas don’t kick in instantly. There’s a background process that checks mailbox sizes periodically. So if a user hits the limit and still manages to send a few emails, don’t panic—it’ll catch up.

Also, if you’re running hybrid, be careful with Exchange Online. The quotas there are tied to the license. E5 gives you 100GB, but if someone gets downgraded to E3, their mailbox might be oversized for the new limit. I’ve had to scramble a few times to archive or export mailboxes before the downgrade.

Lessons Learned

• Monitor proactively: I use a scheduled script to pull mailbox sizes weekly and dump them into a CSV. Helps me spot trends before they become problems.
• Educate users: A quick email every quarter reminding folks to clean up their mailbox goes a long way.
• Don’t rely on defaults: Exchange’s default quotas are fine for small orgs, but once you scale, you’ll need to customize.

Final Thoughts

Mailbox quota management isn’t flashy, but it’s one of those things that saves you from bigger headaches down the line. Whether you’re on-prem, hybrid, or fully in Exchange Online, it pays to stay ahead of mailbox growth.

Ever had a user hit the quota right before a big client pitch? Or found a mailbox so bloated it slowed down your backups? Drop your stories or tips—I’d love to hear how others handle this.

Download the PowerShell Cmdlets used during this video from below.